1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser (VCSEL).
2. Description of the Related Art
Vertical cavity surface emitting lasers are advantageous in terms of their low threshold value, ease of coupling with optical elements, possibility of formation of an array, and the like. Therefore, the VCSELs have been actively studied since the latter half of 1980's.
However, the VCSELs are disadvantageous in that the spot size with which the VCSELs can oscillate in the single transverse mode is small, about 3 to 4 μm in diameter. The reason for this is as follows. When a VCSEL oscillates in the multimode, the responses for an optical element such as a lens are different from each other in each mode, and the emitted light does not behave in the same manner.
Furthermore, since the gain region of the VCSEL is small, a pair of distributed Bragg reflector (DBR) mirrors constituting a resonator must have a high reflectance of 99% or more. In order to achieve such a high reflectance, a multilayer film composed of several tens of layers is necessary in a semiconductor mirror. In such a case, heat is easily accumulated in the resonator because of the large layer thickness of the multilayer film. The unsatisfactory heat dissipation effect increases the threshold value and the electrical resistance, resulting in a difficulty of current injection and the like.
Fan et al. have reported the wavelength dependence of reflected light and transmitted light, etc., when a two-dimensional photonic crystal slab is used as a mirror (V. Lousse et al., Opt. Express, Vol. 12, No. 15, p. 3436 (2004)).
A photonic crystal has a structure in which a refractive index modulation of the order of a predetermined wavelength of light is artificially provided in a material. In other words, in the photonic crystal structure, media having refractive indices different from each other are arrayed with a periodicity. It is believed that the propagation of light in the crystal can be controlled by the multiple scattering effect of the light.
According to the report described in the paper by Fan et al., when light is incident on a surface of a two-dimensional photonic crystal from a direction substantially perpendicular to the surface, light having a predetermined frequency is reflected with an efficiency of about 100%.
For this reason, the present inventors have studied the use of a photonic crystal as a mirror layer of a VCSEL.